In centerless grinding, the workpiece is usually ground between the grinding wheel and the regulating wheel while it rests on a support rail and is rotated. In doing so the regulating wheel and the grinding wheel form a gap which is closed off on the bottom by the support rail to the extent that shaft shaped workpiece is enclosed by way of linear contact with the regulating wheel, the grinding wheel and the supporting surface and its longitudinal axis is thus held in position and rotates. The goal of this configuration is that the workpiece is kept as still as possible in spite of the rotation and the out-of-roundness of the unground raw part.
The raw part that is to be ground is generally subject to a pre-machining before it is finished. The raw part has defects in shape after it is pre-machined, particularly straightness defects and/or radial defects. If a such workpiece with straightness defects is then subjected to a centerless grinding process, the workpiece is first ground at the spot where the point of highest concentric impact lies. Because of such defects in shape, the workpiece does not rest precisely upon the support rail while it is being ground. It is only after it has been fully ground that the entire length of the workpiece will essentially rest on the support rail and it can then—if at all—be ground in a defined and dimension and shape preserving manner.
A centering device, also called a center, is frequently introduced during pre-machining on each end face of the workpiece that is to be ground. This centering is to define the longitudinal axis of the finished workpiece to which the intermediate and the final machining following the pre-machining is to refer. If the workpieces with dimensional defects and defects in shape that have been produced by means of conventional centerless grinding process are then ground, these defects involving the support and/or incomplete support of the workpiece during the grinding of the latter are generally carried over to the finished part. It must however be the goal of centerless grinding that the existing centers on the workpiece are to be disposed concentrically after the grinding operation and/or only deviate from concentricity within very narrow tolerances. It is impossible to guarantee good concentricity of the centers after grinding in the case of known centerless grinding processes. This happens because of the aforesaid problems, on the one hand because of the deficient support of the work piece on the support rail and, on the other hand, because of the production inaccuracies of the workpiece when it is pre-machined.
In a process akin to centerless grinding described in DD 570, the workpiece sits on a prism-shaped groove by way of a linear contact present on each side of the supporting prism and is held in the center range by means of a pressure roller and is pressed into the prism. The known grinding method describes the grinding of two peg-shaped ends of the workpiece. The cones can only exhibit a sufficient concentricity if the workpiece has been accurately ground concentrically before, i.e. its raw contour cannot be retained. The pegs on the ends are ground by means of a grinding wheel without a thrust bearing being present on the side opposite to the grinding wheel. The concentricity accuracy required nowadays cannot be achieved by means of such a process.
DD 119 009 describes a workpiece mounting for centerless grinding of cylindrical parts, in which a grinding slot is defined in a known way by way of a grinding wheel, a regulating wheel and a support rail. A very long cylindrical rod is ground in the grinding slot by means of through feed grinding. To achieve a high concentricity and/or cylindrical shaping of the bar-like rod that is to be ground, the part applied to the support rail is clamped hydrodynamically by pockets or nozzles disposed on the support rail. The pressurization is performed in a controlled way depending on the load during the respective grinding phase. This makes it possible to minimize an out of roundness and/or radial offsets possibly deriving from the pre-machining regarding its effect on the grinding process, but this process would not achieve the desired precision in the case of straightness defects resulting from the pre-machining process.
DE 103 08 292 B4 describes a cylindrical grinding process in the production of tools made of hard metal and a cylindrical grinding machine for grinding raw cylindrical bodies in the production of tools made of hard metal. The tool is fed at the top from an endless hard metal tail stock fed through a chuck of the workpiece headstock. While this process does not involve centerless grinding, this process nevertheless also tries to achieve the highest possible straightness and the lowest possible radial offset after grinding. This is nevertheless achieved in that the a steady rest is ground after mounting and that, after successfully stabilizing the workpiece on the ground steady rest, cylindrical grinding of one of the forward moving ends of the material is performed. The process thus operates “on the moving rod”.
DE 10 2010 010 758 A1 describes a method for topless cylindrical grinding of bar-shaped workpieces and a topless cylindrical grinding machine for grinding such workpieces. In this known process, several individual axially spaced regulating wheels and grinding wheels are alternatingly disposed one behind the other, i.e. stacked, namely with an axial separation such that respective grinding wheels engage in the axial space between opposite regulating wheels and/or regulating wheels in the axial space between opposite grinding wheels. Such an alternating disposition is to minimize a deflection of shaft that is to be ground. The individual grinding wheels are additionally disposed in such a way that the grinding gap becomes gradually narrower in the direction of the exit from the grinding machine. The cylindrical outer contour is thus ground by means of traverse grinding via the individual grinding wheels. The width of the regulating wheels and the grinding wheels in this known cylindrical grinding machine is clearly smaller than the length of the workpiece that is to be ground. The described alternating arrangement makes it possible to grind the entire length of the workpiece at the same time. A controlled light grinding takes place by means of a pre-centering device which is located at the entrance in the throughput direction of the workpiece that is to be ground. This known method does not describe a concentric light grinding of the end regions for purposes of centering the raw part and/or centers. Support discs are instead provided at the entrance and the exit of the device, which serve the purpose of balancing the forces due to the offset and thus non-uniformly operating grinding wheels. So that this force equalization can in fact be achieved, it is necessary, on the one hand, for the width of the regulating wheel to be greater than the width of the grinding wheel and, on the other hand, for the spindles for the regulating wheels and the grinding wheels to have sturdy dimensions and a small axial gap between the overlap regions of the regulating wheels and the grinding wheels must be realized.
The Mikrosa Company is known as manufacturers of centerless grinding wheels. It uses a method in which the workpiece is ground between so-called auxiliary centers and then, after the centers are loosened, the same workpiece is subjected to centerless finish-grinding at the same grinding station while sitting on a support rail between the grinding wheel and the regulating wheel. Both the technical formation and the alignment of the centers requires a relatively high expenditure and the entire system is difficult to control as regards precision.